The present invention relates to a carrier material for receiving material for dye diffusion thermal transfer (D2T2), which comprises a carrier and an intermediate layer.
In recent years a method of dye diffusion thermal transfer has been developed which makes possible the reproduction of an electronically produced picture in the form of a "hardcopy." The principle of such a method is as follows. A digital picture is prepared with respect to the basic colors cyan, magenta, yellow and black and it is converted to the corresponding electronic signals, which then are converted into heat by a thermohead. Due to the effects of the heat, the dye sublimes out of the donor coating of a color strip or sheet that is in contact with the receiving material and the dye diffuses into a receptor layer.
The receiving material for the thermal dye transfer as a rule consists of a carrier with a receptor layer applied to its front side. Aside from the receptor layer, other layers are often also applied to the front side of the carrier. These include, for example intermediate layers, such as barrier, separation and adhesive layers among others, or protective layers. Plastic film, for example polyester film or a coated paper, can be used as carriers.
The principal component of the receptor layer as a rule, is a thermoplastic resin that has an affinity for dye from the color ribbon. Plastics with ester compounds can be used for this, for example polyester resins, polyacrylate resins, polycarbonate resins, polyvinyl acetate resins and styrene acrylate resins, plastics with amide bonds, for example polyamide resins, as well as mixtures of the resins listed. However, copolymers that have at least one of the above named structures as a principal component, for example vinyl chloride/vinyl acetate copolymer, may also be used.
To obtain pictures with higher quality with respect to optical density, color tone (reproducibility of graduation) and resolution, the following requirements are set for the receiving material:
smooth surface; PA1 heat stability; PA1 light stability; PA1 good dye solubility; PA1 good resistance to scratching and rubbing; PA1 "anti-blocking" characteristics (no sticking).
It is known that in spite of achieving the above named qualities, qualitatively inferior pictures can occur which can be traced to an insufficient contact of the thermohead on the opposing transport roller in the printer, and which results in unprinted portions. To prevent this effect, a further requirement is set for the receiving material, which requirement is a so-called softness. A soft, smearable receiving material can, for example, be produced by applying an intermediate coating that fulfills the function of a cushion coating.
This problem is supposedly solved in JP 62-146693 by the application of a cushion layer consisting of styrene/butadiene or vinyl acetate latex.
In another patent JP 02-274592 an intermediate layer of foamy polypropylene is applied.
The same problem is supposed to have been solved in another patent JP 03-092382 by the application of a microporous resin intermediate layer.
Furthermore, to include a pellet shaped filler, such as polypropylene, in a cushion layer, is known from JP 03-110195.
In DE 3,934,014, a receiving material is proposed in which a porous heat insulated layer that contains macromolecular micropellets in the form of hollow resin particles and/or heterogeneous resin particles is applied to a substrate.
The disadvantage of this receiving material is the porosity of the intermediate coating. Because of this porosity, dye can leak out of the receptor layer into the interior of the base sheet, and the transferred picture appears blurry.
The objective of the invention comprises making available a carrier material for a receiving material for the thermal sublimation process which, independent of the type and composition of the image receptor layer makes possible the printing of pictures with high color density and resolution with an even distribution of the dye on the surfaces and without unprinted portions.
This objective is attained by means of an intermediate layer which contains a film forming binder with a minimum film forming temperature (MFFT) of at least 25.degree. C. and a pigment in the form of hollow polymer micropellets in which the micropellets have an interior space that has a volume that is between 10 and 55% of the total volume of the pellet body. Particularly suitable are micropellets whose interior volume is 12.5-25% of the total pellet body.
The hollow micropellets have a diameter of 0.4-1 .mu.m, and preferably 0.4-0.6 .mu.m.
The intermediate layer according to the invention is applied to a base paper coated with polyolefin, in particular, a base paper coated with polyethylene or polypropylene.
The material of the micropellets is selected from styrene, acrylic, and/or styrene/acrylic copolymer resins.
Contrary to the effect that would be anticipated, it has been surprisingly shown that with a quantity of 4-30 wt % of micropellets in the intermediate layer, a high resolution of the pictures transferred, as well as their even appearance without unprinted portions, can be obtained.
Furthermore, by the use of the micropellets according to the invention, a good opacity of the material coated with the intermediate layer is obtained, which opacity also hides the usual markings on the back side.
By the use of the intermediate layer according to the invention, a good barrier effect is obtained between the receptor layer and the paper carrier coated with polyolefin. In this way, during the effects of the heat, dyes do not diffuse into the paper base and are not carried farther by materials from the polyolefin coating, which would result in a blurry appearance of the picture.
The film forming binder used in the intermediate layer according to the invention with a minimum film forming temperature of at least 25.degree. C. is a resin that is soluble in organic solvents and which resin is from the group of acrylonitrile, acrylate, vinyl chloride, vinyl acetate, vinylidene chloride, polyamide, urethane homopolymers or copolymers as well as mixtures of these resins. Aside from the listed binders, a mixture that always contains polyvinylidene chloride, such as a mixture of acrylate copolymer and polyvinylidene chloride, and whose minimum film forming temperature is approximately 26.degree. C. has been shown to be particularly advantageous.
Furthermore, the intermediate layer can also contain other additives, such as dispersing agents, release agents, dyes and other additives.
The intermediate coating is applied to the carrier as an aqueous dispersion using any useful application and metering procedures, such as for example doctor blades, rollers, brushes, gravure or nip processes, and is subsequently dried. The coating weight of the dried layer is 0.5-50 g/m.sup.2 and preferably 2-10 g/m.sup.2.
In a particular embodiment of the invention, a layer is applied to the back side of the receiving material to prevent the imprinting of the picture on the back side of the material. This back side layer can contain binders, such as starch, gelatin, and other adjuvants, such as pigments.
The invention will be explained using the following examples.